Recognition that the macroscopic properties of materials depend not only on their chemical composition, but also on the size, shape and structure has spawned investigations into the control of these parameters for various materials. In this regard, the fabrication of uniform hollow spheres has recently gained much interest. Hollow capsules with nanometer and micrometer dimensions offer a diverse range of potential applications, including utilization as encapsulants for the controlled release of a variety of substances, such as drugs, dyes, proteins, and cosmetics. When used as fillers for coatings, composites, insulating materials or pigments, hollow spheres provide advantages over the traditional solid particles because of their associated low densities. Hollow spheres may also be used in applications as diverse as hierarchical filtration membranes and proppants to prop open fractures in subterranean formations.
Ceramic proppants are widely used as propping agents to maintain permeability in oil and gas formations. Conventional proppants offered for sale exhibit exceptional crush strength but also extreme density. Typical densities of ceramic proppants exceed 100 pounds per cubic foot. Proppants are materials pumped into oil or gas wells at extreme pressure in a carrier solution (typically brine) during the fracturing process. Once the pumping-induced pressure is removed, proppants “prop” open fractures in the rock formation and thus preclude the fracture from closing. As a result, the amount of formation surface area exposed to the well bore is increased, enhancing recovery rates. Proppants also add mechanical strength to the formation and thus help maintain flow rates over time. Three grades of proppants are typically employed: sand, resin-coated sand and ceramic proppants. Proppants are principally used in gas wells, but do find application in oil wells.
Relevant quality parameters include: particle density (low density is desirable), crush strength and hardness, particle size (value depends on formation type), particle size distribution (tight distributions are desirable), particle shape (spherical shape is desired), pore size distribution (tight distributions are desirable), surface smoothness, corrosion resistance, temperature stability, and hydrophilicity (hydro-neutral to phobic is desired).
Ceramic proppants dominate sand and resin-coated sand on the critical dimensions of crush strength and hardness. They offer some benefit in terms of maximum achievable particle size, corrosion and temperature capability. Extensive theoretical modeling and practical case experience suggest that conventional ceramic proppants offer compelling benefits relative to sand or resin-coated sand for most formations. Ceramic-driven flow rate and recovery improvements of 20% or more relative to conventional sand solutions are not uncommon.
Ceramic proppants were initially developed for use in deep wells (e.g., those deeper than 7,500 feet) where sand's crush strength is inadequate. In an attempt to expand their addressable market, ceramic proppant manufacturers have introduced products focused on wells of intermediate depth.
Resin-coated sands offer a number of advantages relative to conventional sands. First, resin coated sands exhibit higher crush strength than uncoated sand given that resin-coating disperses load stresses over a wider area. Second, resin-coated sands are “tacky” and thus exhibit reduced “proppant flow-back” relative to conventional sand proppants (e.g. the proppant stays in the formation better). Third, resin coatings typically increase sphericity and roundness thereby reducing flow resistance through the proppant pack.
Ceramics are typically employed in wells of intermediate to deep depth. Shallow wells typically employ sand or no proppant. As will be described in later sections, shallow “water fracs'” represent a potential market roughly equivalent to the current ceramic market in terms of ceramic market size.
With a combined annual production of over 30 million tons, the oxides and hydroxides of aluminum are undoubtedly among the most industrially important chemicals (K. Wefers and C. Misra, “Oxides and Hydroxides of Aluminum.” Alcoa Laboratories, 1987). Their uses include: precursors for the production of aluminum metal, catalysts and absorbents; structural ceramic materials; reinforcing agents for plastics and rubbers, antacids and binders for the pharmaceutical industry; and as low dielectric loss insulators in the electronics industry. With such a diverse range of applications, it is unsurprising that much research has been focused on developing and understanding methods for the preparation of these materials.
Traditional ceramic processing involves three basic steps generally referred to as powder processing, shape forming, and densification, often with a final mechanical finishing step. Although several steps may be energy intensive, the most direct environmental impact arises from the shape-forming process where various binders, solvents, and other potentially toxic agents are added to form and stabilize a solid (“green”) body. In addition to any innate health risk associated with the chemical processing, these agents are subsequently removed in gaseous form by direct evaporation or pyrolysis. In many cast-parts, the liquid solvent alone consists of over 50% of the initial volume of material. The component chemicals listed, with relative per percentage, in Table 1 are essentially mixed to a slurry, cast, then dried and fired. All solvents and additives must be removed as gaseous products via evaporation or pyrolysis.
TABLE 1Composition of a non aqueous tape-casting alumina slurryFunctionCompositionVolume %PowderAlumina27Solvent1,1,1-Trichloroethylene/Ethyl Alcohol58DeflocculentMenhaden Oil1.8BinderPolyvinyl Butyrol4.4PlasticizerPolyethylene Glycol/Octyl Phthalate8.8
Whereas the traditional sintering process is used primarily for the manufacture of dense parts, the solution-gelation process has been applied industrially primarily for the production of porous materials and coatings. Solution-gelation involves a four-stage process: dispersion; gelation; drying; firing. A stable liquid dispersion or sol of the colloidal ceramic precursor is initially formed in a solvent with appropriate additives. By change in concentration (aging) or pH, the dispersion is polymerized to form a solid dispersion or gel. The excess liquid is removed from this gel by drying and the final ceramic is formed by firing the gel at higher temperatures.
The common solution-gelation route to aluminum oxides employs aluminum hydroxide or hydroxide-based material as the solid colloid, the second phase being water and/or an organic solvent. Aluminum hydroxide gels have traditionally been prepared by the neutralization of a concentrated aluminum salt solution; however, the strong interactions of the freshly precipitated alumina gels with ions from the precursors solutions makes it difficult to prepare these gels in pure form. To avoid this complication alumina gels may be prepared from the hydrolysis of aluminum alkoxides, Al(OR)3 (Eq. 1).
Although this method was originally reported by Adkins in 1922 (A. Adkins, J. Am. Chem. Soc. 1922, 44, 2175), it was not until the 1970's when it was shown that transparent ceramic bodies can be obtained by the pyrolysis of suitable alumina gels, that interest increased significantly (B. E. Yoldas, J. Mat. Sci. 1975, 10, 1856).
The exact composition of the gel in commercial systems is ordinarily proprietary, however, a typical composition can include an aluminum compound, a mineral acid and a complexing agent to inhibit premature precipitation of the gel, e.g., Table 2. The aluminum compound was traditionally assumed to be the direct precursor to pseudo-boehmite. However, the gel is now known to consist of aluminum-oxygen macromolecular species with a boehmite-like core: alumoxanes.
TABLE 2Typical composition of an alumina sol-gelfor slipcast filter membranesFunctionCompositionBoehmite PrecursorASB [aluminum sec-butoxide, Al(OC4H9)3]ElectrolyteHNO3 0.07 mole/mole ASBComplexing agentglycerol ca. 10 wt. %
The replacement of 1,1,1-trichloroethylene (TCE) as a solvent in the traditional ceramic process must be regarded as a high priority for limiting environmental pollution. Due to its wide spread use as a solvent in industrial processes, TCE has become one of the most commonly found contaminants in ground waters and surface waters. Concentrations range from parts per billion to hundreds of milligrams per liter. The United States Environmental Protection Agency (USEPA) included TCE on its 1991 list of 17 high-priority toxic chemicals targeted for source reduction. The 1988 releases of TCE reported under the voluntary right to know provisions of Superfund Amendments and Reauthorization Act (SARA) totaled to 190.5 million pounds.
The plasticizers, binders, and alcohols used in the process present a number of potential environmental impacts associated with the release of combustion products during firing of the ceramics, and the need to recycle or discharge alcohols which, in the case of discharge to waterways, may exert high biological oxygen demands in the receiving communities.
Ceramic ultrafiltration (UF) and nanofiltration (NF) membranes have been fabricated by the sol-gel process in which a thin membrane film is deposited, typically by a slip-cast procedure, on an underlying porous support. This is typically achieved by hydrolysis of Al, Ti, Zr or other metal compounds to form a gelatinous hydroxide at a slightly elevated temperature and high pH. In the case of alumina membranes, this first step may be carried out with 2-butanol or iso-propanol. After removing the alcohol, the precipitated material is acidified, typically using nitric acid, to produce a colloidal suspension. By controlling the extent of aggregation in the colloidal sol, membranes of variable permeability may be produced. The aggregation of colloidal particles in the sol is controlled by adjusting the solution chemistry to influence the diffuse layer interactions between particles or through ultrasonification. Alternatively, a sol gel can be employed, which is then applied to a porous support. While this procedure offers greater control over membrane pore size than does the metal precipitation route, it is nonetheless a difficult process to manipulate. In both cases, plasticizers and binders are added to improve the properties of the slip cast solution. Once the film has been applied it is dried to prevent cracking and then sintered at high temperature.
The principal environmental results arising from the sol-gel process are those associated with use of strong acids, plasticizers, binders, and solvents. Depending on the firing conditions, variable amounts of organic materials such as binders and plasticizers may be released as combustion products. NOx's may also be produced from residual nitric acid in the off-gas. Moreover, acids and solvents must be recycled or disposed of. Energy consumption in the process entails “upstream” environmental emissions associated with the production of that energy.
The aluminum-based sol-gels formed during the hydrolysis of aluminum compounds belong to a general class of compounds: alumoxanes. Alumoxanes were first reported in 1958 and have since been prepared with a wide variety of substituents on aluminum. The structure of alumoxanes was proposed to consist of linear (I) or cyclic (II) chains (S. Pasynkiewicz, Polyhedron, 1990, 9, 429). Recent work has redefined the structural view of alumoxanes, and shown that they are not chains but three dimensional cage compounds (A. W. Apblett, A. C. Warren, and A. R. Barron, Chem. Mater., 1992, 4, 167; C. C. Landry, J. A. Davis, A. W. Apblett, and A. R. Barron, J. Mater. Chem., 1993, 3, 597). For example, siloxy-alumoxanes, [Al(O)(OH)x(OSiR3)1-x]n, consist of an aluminum-oxygen core structure (III) analogous to that found in the mineral boehmite, [Al(O)(OH)]n, with a siloxide substituted periphery.

Precursor sol-gels are traditionally prepared via the hydrolysis of aluminum compounds (Eq. 1). This “bottom-up” approach of reacting small inorganic molecules to form oligomeric and polymeric materials has met with varied success, due to the difficulties in controlling the reaction conditions, and therefore the stoichiometries, solubility, and processability, of the resulting gel. It would thus be desirable to prepare alumoxanes in a one-pot bench-top synthesis from readily available, and commercially viable, starting materials, which would provide control over the products.
In the siloxy-alumoxanes, the “organic” unit itself contains aluminum, i.e., IV. Thus, in order to prepare the siloxy-alumoxane similar to those previously reported the anionic moiety, the “ligand” [Al(OH)2(OSiR3)2]−, would be used as a bridging group; adding this unit would clearly present a significant synthetic challenge. However, the carboxylate-alumoxanes represent a more realistic synthetic target since the carboxylate anion, [RCO2]−, is an isoelectronic and structural analog of the organic periphery found in siloxy-alumoxanes (IV and V). Based upon this rational, a “top-down” approach has been developed based upon the reaction of boehmite, [Al(O)(OH)]n, with carboxylic acids, Eq. 2 (Landry, C. C.; Pa ppè, N.; Mason, M. R.; Apblett, A. W.; Tyler, A. N.; MacInnes, A. N.; Barron, A. R., J. Mater. Chem. 1995, 5, 331).

The carboxylate-alumoxane materials prepared from the reaction of boehmite and carboxylic acids are air and water stable materials and are very processable. The soluble carboxylate-alumoxanes can be dip-coated, spin coated, and spray-coated onto various substrates. The physical properties of the alumoxanes are highly dependent on the identity of the alkyl substituents, R, and range from insoluble crystalline powders to powders that readily form solutions or gels in hydrocarbon solvents and/or water. The alumoxanes are indefinitely stable under ambient conditions, and are adaptable to a wide range of processing techniques. Given the advantages observed for the application of carboxylate alumoxanes, e.g., the low price of boehmite ($1 kg−1) and the availability of an almost infinite range of carboxylic acids make these species ideal as precursors for ternary and doped aluminum oxides. The alumoxanes can be easily converted to γ-Al2O3 upon mild thermolysis (A. W. Apblett, C. C. Landry, M. R. Mason, and A. R. Barron, Mat. Res. Soc., Symp. Proc., 1992, 249, 75).